The present disclosure relates to a developing device used in an image forming apparatus such as a printer and particularly to a developing device adopting a two-component developer containing a carrier and a toner and an image forming apparatus provided with the same.
Conventionally, a developing device as described below is known as a developing device used in an image forming apparatus such as a printer. Such a developing device is formed by housing a screw feeder (agitating member), a magnetic roller and a layer thickness restricting member in a development housing. The screw feeder agitates a developer by rotating about a shaft center. The magnetic roller is arranged parallel to this screw feeder and supplies a toner fed from the screw feeder to the circumferential surface of a photoconductive drum by rotating about a shaft center. The layer thickness restricting member extends in an axial center direction of the magnetic roller and restricts the amount of the developer on the magnetic roller. A leading edge part of the layer thickness restricting member faces the circumferential surface of the magnetic roller.
Here, by rotating the screw feeder about the shaft center, the developer loaded in a case is moved upward while being agitated, and compressed through a clearance between a compressing member arranged to face the screw feeder and the screw feeder (hereinafter, referred to as a developer compressing clearance). Thereafter, this developer passes between the layer thickness restricting member and the magnetic roller and is supplied to the circumferential surface of the magnetic roller in a state set to a predetermined thickness. Since the developer is smoothly fed toward the layer thickness restricting member while being kept in a compressed state by the presence of this developer compressing clearance, there is no such inconvenience that the developer moves toward the layer thickness restricting member in an insufficiently compressed state.
The above effect is achieved by the conventional technology under such a condition that the amount of the developer in the development housing is relatively small and the developer separated from the magnetic roller and having fallen down is conveyed upward again after slipping under the screw feeder since the developer can pass through the developer compressing clearance. However, if the amount of the developer in the development housing is relatively large, the following problem occurs. That is, if the developer is stored in the development housing to such a degree as to cover an area above the screw feeder, the developer separated from the magnetic roller and having fallen down cannot slip under the screw feeder after passing a developing portion in which the developer is supplied toward the photoconductive drum. Thus, the developer that has fallen, triggered by the operation of the screw feeder, may adhere to the magnetic roller again. Since a toner/carrier ratio differs between the fallen developer and the developer in the development housing, a toner density distribution is produced on the magnetic roller, with the result that the density of an image formed on a sheet may vary. In such a case, the separated developer cannot pass through the developer compressing clearance with the conventional technology, wherefore it is difficult to solve a variation of toner density on the magnetic roller.
The present disclosure was developed to solve the problem as described above and an object thereof is particularly to solve a variation of toner density on a magnetic roller of a developing device.